This application claims the benefit of PCT/DK00/00493 filed Sep. 7, 2000 and Danish Application No. PA 1999 01260 filed Sep. 8, 1999.
The present invention concerns a refrigerator as described in the preamble of claim 1.
In the known, high-efficiency refrigerating units, according to prior art there is used drop separators between the evaporator and the suction pipe of the compressor. The drop separator is used for preventing liquid drops of refrigerant from reaching the compressor which would be destructive to the mechanical parts of the compressor, especially the valves. Besides, it is suitable for attaining maximum efficiency that the gas, when leaving the drop separator and is conducted to the compressor via the suction pipe, is dry, i.e. it does not contain refrigerant in the liquid state.
The drop separator works in such a way that the gas and possible liquid drops from the evaporator are sucked into the drop separator where the drops, due to gravitation, fall down to the bottom of the drop separator, after which the dry gas may be sucked to the compressor from the upper part of the drop separator. In order that the separation of gas from the liquid drops is efficient, it is required that the drop separator is relatively large, something which is a disadvantage in general. Furthermore, the use of a large drop separator implies the use of a corresponding large amount of refrigerant which is a further disadvantage as the refrigerant is often poisonous and/or harmful to the environment.
Reduction of the size of a refrigerating unit with corresponding reduction of the amount of the refrigerant as consequence may be achieved by using a centrifugal separator instead of a drop separator. A centrifugal separator functions by the mixture of gas and liquid drops being supplied to a cylindric container in approximately tangential direction. The supply results in a rotating whirl in the cylinder whereby liquid is flung to the rim of the cylinder where it runs down to the bottom of the cylinder, whereafter the dry gas may be sucked out of the central area of the cylinder in the uppermost part of the cylinder.
In Danish laid-open publication DK 147133 B is described a refrigerator using a cyclone separator between the evaporator and an enclosed compressor. The intention is to prevent liquid drops and dirt from getting into the compressor. After separation of the liquid, it is returned to the interior of the enclosure. The refrigerant is here mixed with compressor oil and returned to the system after evaporating the refrigerant from the oil sump in the compressor enclosure.
Furthermore, the refrigerator according the above prior art has the drawback that the cooling efficiency fluctuates during operation which is a great disadvantage in connection with ice cream machines. In ice cream machines it is decisive for the quality of the ice cream that cooling occurs with constant efficiency during the process.
In Danish patent publication DK 74847 is described another design which reduces the necessary amount of refrigerant. The reduction does not, however, depend on a reduction of the size of the liquid separator but is provided by abnormal flow conditions in the evaporator as the liquid is taken in at the top of the evaporator and the sucking occurs lowermost. Thereby accumulation of liquid in the evaporator is avoided whereby the amount of refrigerant is reduced. The design may only, however, function with evaporators shaped as tube batteries and may therefore not be used with flooded evaporators as for example those used in ice cream refrigerators.
European patent application EP 217 605 describes a control method for maintaining a constant liquid level in a refrigerant tank in a refrigerating unit. Here, the controlling occurs on the basis of signals from two level sensors where a step motor influences and sets a valve with varying nozzle area This system has the drawback that the liquid injection is not controlled by the liquid level in the evaporator but by the level in a remote accumulator without any liquid connection between accumulator and evaporator. Thereby a constant liquid level in the evaporator is not ensured. For example, in ice cream refrigerators a constant liquid level in the evaporator is, however, an indispensable necessity in order to achieve a stable operation.
The purpose of the present invention is to indicate a refrigerator having the known advantages achieved by using a cyclone separator, and which does not have the above mentioned disadvantages.
This purpose is achieved by the present invention by a refrigerator of the type indicated in the introduction, and which is designed with the features described in the characterising part of claim 1.
By using a cyclone separator in the refrigerator according to the invention there is achieved a relatively small refrigerator as the large volume required by a conventional drop separator in the refrigerator may be avoided. This is an advantage during production of refrigerators and also in use.
Due to the reduction of the refrigerator volume by using a cyclone separator, the necessary amount of refrigerant is also much lesser. For example, the typical refrigerant volume ratio between units with drop separator and units with cyclone separator is 12:1. Considering that certain refrigerants, for example ammonia, freon, or propane, are poisonous and/or environmentally harmful by leakage compared with the fact that refrigerators are often located in buildings in which people are working, the reduced amount of refrigerant is a great advantage.
For ensuring a constant efficiency in the refrigerator, four mutually interdependent circumstances and each contributing to equalising fluctuations in the cooling efficiency will be described in the following.
The liquid separated in the cyclone separator is returned to the evaporator via the down pipe in the cyclone separator. The liquid level in the cyclone separator is equal to the liquid level in the delivery pipe above the evaporator. This liquid level is controlled by a level sensor, for example a vibration level switch, in the cyclone separator registering whether the fluid around its sensors is gas or liquid. In the case that the sensor registers liquid in the cyclone separator it is recognised that the evaporator is filled with refrigerant as the sensor is disposed at a distance above the evaporator.
In case the sensor in the cyclone separator does not register any liquid which means that the liquid level in the delivery pipe and possibly the evaporator falls, further refrigerant is supplied to the evaporator via a valve. This refrigerant is substantially returned liquid from the condensator in the refrigeration circuit. In this way it is ensured that the evaporator is always filled with liquid which is one of the circumstances of the invention improving the refrigerator with respect to a more uniform efficiency.
The said valve for supplying refrigerant from the condensator to the evaporator is preferably electronically controlled and has a modulating opening so that the liquid supply to the evaporator occurs in a more even way instead of by discrete portions of a certain size.
If liquid is supplied in portions, there is danger that the liquid circulation in the system starts oscillating which may result in fluctuating efficiency of the refrigeration circuit. The very even supply of refrigerant from the condensator to the evaporator is another circumstance improving the refrigerator with respect to more even efficiency.
Alternatively, the liquid supply is governed by a mechanical device, for example a float connected to a valve, where this valve is characterised by having an opening area varying evenly with the position of the float so that the float may adjust itself according to the actual liquid need while the valve gives an approximately even flow.
The supply of refrigerant from the condensator to the evaporator occurs via an ejector provided in the down pipe. In this way increased circulation is created in the system, improving the efficiency. Besides, the increased circulation counteracts building up of oscillations in the system, contributing to equalisation of the efficiency.
By a further development of the invention, the liquid supply is controlled directly to the cyclone separator via a pipe discharging tangentially in the cyclone. The liquid supply pipe is mounted so that the direction of rotation of the injected liquid is the same as for the gas in the cyclone separator. Hereby further advantages are achieved. First, the rotation of the gas is supported, resulting in a more efficient liquid separation. Second, a faster separation of the so-called flash-gas, that may be formed in the liquid by injection, is achieved. Third, the injected liquid is very quickly collected in the cyclone separator 21 whereby delays between injection and level measuring in the are avoided. Thereby, this device works as a further factor in preventing the arise of oscillations.